The objective of this proposal is to determine how extracellular lipoprotein bound cholesterol enters adrenal steroid producing cells. The aspects of the problem will be examined: cholesterol membrane translocation; lipoprotein and apoprotein binding to cell membranes and intracellular organelles; and the effects of extracellular lipoproteins on intracellular cholesterol synthesis and cholesterol ester hydrolysis. Preliminary studies suggest that rat adrenal tissue accumulates HDL cholesterol in response to ACTH stimulation and that apoA-I participates in the uptake phenomenon. In addition, a reversible binding site for 125I apoA-I has been observed. The current proposal is aimed at obtaining a better understanding of these processes and their effect on adrenal intracellular cholesterol metabolism. In order to understand the mechanism of uptake, we will examine net cholesterol flux from HDL into rat adrenal cells in suspension or adrenal tumor cells in culture. The effects of ACTH as well as added apoA-I or apoA-II on this process will be examined. Uptake of iodonated HDL and HDL labelled with 32p-lecithin will also be studied. Secondly the binding of 125I apoA-I to rat adrenal tissue will be further characterized by determining the subcellular location of binding as well as the effects of pH, ionic strength, and chemical modification of apoA-I on the process. Finally the effects of extracellular lipoproteins on adrenal cholesterol synthesis and cholesterol esterase activity will be examined. These studies should provide insight into lipoprotein-cellular interactions in general and into HDL structure and function in particular. This information may be useful not only in gaining a further understanding of atherosclerosis but also in gaining insight into adrenal function.